Dot matrix line printers are well known in the industry. Dot matrix line printers have an elongated bank of hammers forming a print head. The print head is shuttled back and forth over a small number of character positions during printing. That is, rather than shuttling a small print head back and forth over the entire width of the paper during printing, the print head of a dot matrix line printer is wide and shuttled only over a few character positions. For example, the print head may include 66 dot printing elements located along a linear axis or line. Each of the dot printing elements is shuttled over two character positions. Thus, back and forth shuttling of the print head allows a maximum of 132 characters to be printed. As the print head is shuttled, the dot printing elements are selectively impacted to create dots. A series of lines of dots creates a row of characters or, alternatively, a graphic image.
Dot matrix line printers are often used to print upon a continuous web of print receivable media, such as paper, fabric, metal, synthetic materials, organic materials, etc. The continuous web of print receivable media may be formed from a series of sheets of paper coupled to one another in a head-to-toe (i.e., top-to-bottom) relationship. The sheets may be blank or may have forms pre-printed thereon. The sheets may also include two or more layers, such that one or more carbon copies may be simultaneously created by printing upon the top sheet.
Typically, the paper includes a pair of detachable drive strips coupled to the sides of the paper. The drive strips include a series of evenly spaced perforations designed to be engaged by a series of pins of a pair of drive tractors. The drive tractors engage the perforations in the drive strips to drive (move) the paper past the print head.
Many previously developed dot matrix line printers use two pairs of drive tractors to move paper and control paper tension when printing upon the paper. More specifically, such previously developed printers include a first pair of tractors (one tractor located on each side of the paper) located downstream of the print head, and a second pair of tractors located upstream of the print head. The two pairs of tractors work in tandem to move the paper across the print head while maintaining an appropriate amount of tension in the paper. Recently, in order to reduce the manufacturing costs of line printers, printers that employ only one pair of tractors to move the paper across the print head have been developed. Due to the difficulty of pushing paper through the narrow gap between a print head and a platen, the tractors of such printers are located downstream of the print head. As a result, the tractors pull the paper past the print head.
When only a single pair of drive tractors are used, paper tension becomes an important issue. It is important for good print quality that paper be kept taut during printing. Consistent paper tension may be provided in single tractor dot matrix line printers by a paper iron. A paper iron usually includes a cantilevered strip of spring material that pinches paper against the platen upstream of the print head. The paper iron is biased by the spring towards the platen so as to apply a consistent paper pinching force. The friction applied to the paper by the paper iron and the platen provides controlled, consistent paper tension.
Typically, the sheets forming a continuous web of paper are printed in a continuous manner. As a result, after one sheet is printed, the next sheet is printed without the first sheet being detached from the web. However, on occasion, a user may wish to detach the first sheet for use or review prior to the printing of the second sheet. This type of printing is often called single sheet printing.
In the past, when a printer having a single pair of pull drive tractors performed single sheet printing, the print could be no closer to the top edge of a sheet than the distance between the closest tractor pin and the print hammers of the print head. Due to the presence of a ribbon shield above the print hammers, this distance is usually greater than one inch. If printing is required closer than one inch to the top edge of the sheet, a sacrificial sheet must precede the sheet to be printed. The sacrificial sheet is used to preserve tractor engagement with the paper while the printer prints upon the sheet to be printed. Thus, two sheets are required for every one that is printed. Such printing can be expensive, especially if the sheets are preprinted forms or carbon copies are to be produced. A 50% print yield can significantly increase costs for single sheet printing employing single tractor dot matrix line printers and other types of printers, in particular impact printers, employing a single pair of tractors.
Therefore, there exists a need for single tractor dot matrix line and other printers and methods of printing employing such printers that provide for single sheet printing near the top and/or bottom margins of single sheets without requiring a sacrificial sheet.